1. Field of the Invention
The invention relates to an electric heater with at least one spirally arranged heating resistor, and in particular to an improved arrangement wherein a plurality of heating resistors are disposed uniformly on a cooking surface and are switchable in power steps.
Heaters with spirally arranged heating resistors as known, for example, from U.S. Pat. No. 4,538,051--Schreder et al, are mainly used as radiant heaters for the cooking points of cookers. The cooking surface defined by the complete heating field of the heater is heated in different cooking fields during each operational switching position. For example, the resistors are powered at each power setting of the heater as a whole, and not individually, as would also be possible. The heater can also be provided for heating a baking oven muffle, a cast metal hotplate body of an electric hotplate, or some other wall to be heated. In the case of construction as a radiant heater and not as conductive a heater, whose heating resistors are completely embedded in a pressed insulating material, the heater is appropriately arranged on the back of a transparent plate, e.g. a glass ceramic plate. The standard arrangement of the heating resistors is such that using a so-called seven step timing switch, it is possible selectively to switch three heating resistors between a minimum and a maximum total power through a randomly selected individual, series and/or parallel connection of the resistors defining six power stages, the seventh stage being used for the complete disconnection of all the heating resistors. In general, the heating resistors have different rated power levels, so that a heating resistor to be operated just below the average total power, e.g. in the fourth power stage in individual connection has the highest rated power. A heating resistor to be operated in a power stage below it, e.g. the third stage in individual connection has an average rated power, and the third heating resistor has the lowest rated power. If the three heating resistors are only arranged in two spirals, i.e. in a double thread, then for specific power settings in which one or more heating resistors are completely disconnected, there can be a relatively non-uniform spatial distribution of the heating in such a way that heating power is only obtained in the peripheral area or only in the central area of the overall heating field, which although desirable in certain cases, is not appropriate in others.
2. Summary of the Invention
An object of the present invention is to provide a heater of the aforementioned type in which, in simple manner, it is possible to make the length of at least one first heating resistor greater than would be possible by laying or wiring in a single spiral, so that as a result of said length this heating resistor can have a relatively high rated power, but still ensures a problem-free laying or wiring in the heating field.
According to the invention this object is achieved in the case of an electric heater of the aforementioned type in that at least one heating resistor forms at least two spirals with nested spiral turns. In addition thereto or instead thereof one of these spirals formed by said first heating resistor can be extended over and beyond the spiral shape at the periphery and/or in the center of the heating field. A particularly advantageous arrangement is obtained if the first heating resistor forms an outer spiral and an inner spiral, the outermost turn of the outer spiral externally surrounding all the remaining spiral turns of said first heating resistor. It would also be conceivable for the first heating resistor to form three or more, and in particular also nested spirals, so that the heating resistor length would be increased and it would also be possible to further improve its uniform distribution of heat over the entire heating field. If the heating resistor comprises a wire coil, then despite the high rated power the wire cross-section can be made relatively small and it is possible to provide a relatively large coil pitch, which is advantageous with respect to the life of the heating resistor.
It is also possible to place two or more heating resistors in two or more spirals. However, generally the heating resistors are not then all arranged continuously in a common plane over their entire length and instead between adjacent portions or spirals of the heating resistor are provided electrically conductive connecting bridges, which are displaced towards the back or front of said heating resistor with respect to said portions and in contact-free manner pass around the remaining heating resistor or resistors. A particularly advantageous construction is obtained if only a single heating resistor, particularly that with the highest rated power, is lengthened in the described way, because then between its two ends provided for the electrical connection it can be uninterruptedly continuous and identically constructed over its entire length and between its ends forms a length portion, which connects a spiral to the following, in particular also spiral portion.
In place of the described construction, but in particular in addition thereto, the invention also provides with respect to a heating resistor the embedding in an insulating body of spaced longitudinal portions and interposed longitudinal portions substantially in exposed manner and in particular in portions of a spiral slot. Appropriately the insulating support is provided with rib-like or stud-like protuberances shaped therefrom and running in spaced manner in the longitudinal direction of the heating resistor. The heating resistor or the heating coil is embedded over part of the helical circumference in the protuberance, whereby the particular heating resistor can be roughly half embedded in the vicinity of the protuberances and the area of the central axis of the heating coil is substantially free from embedding in such a way that the inner circumference of the heating coil does not have to be completely covered by the insulating body material in the vicinity of the embedding and can instead directly emit heat.
In a particularly advantageous manner the protuberances can be formed in that between adjacent protuberances in the longitudinal direction of the heating resistor is formed in each case one portion of the spiral slot, so that the protuberances do not have to project beyond the front or the front surface of the insulating body and instead said front surface together with the heat surfaces of the protuberances are located substantially in one plane. In order that at least partly exposed longitudinal portions of the heating resistor can be provided, said front surface is then hollowed out by corresponding depressions in the vicinity of said longitudinal portions, said depressions forming the portions of the spiral slot. However, the construction can also be in accordance with German patent 27 29 929, in which the protuberances receiving the heating resistors project beyond the front of the insulating body and reference should be made thereto for further details and effects.
These and further features of preferred further developments of the invention can be gathered from the claims, the description and drawings, whereby the individual features can be realized in an embodiment of the invention and in other fields either individually or in the form of subcombinations and can represent advantageous and optionally independently patentable constructions, for which protection is hereby claimed.